A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art displays. CRT displays typically use a layer of phosphor that is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams that generate electrons that are scanned across the phosphor in a raster pattern. The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum tubes that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, typically, other types of display technologies such as active matrix liquid crystal display, plasma display and electroluminiscent display technologies have been used in the past to form thin displays.
Recently, a thin flat panel display has been developed that uses the same process for generating pictures as is used in CRT devices. These thin flat panel displays use a backplate including a matrix structure of rows and columns of electrodes. One such flat panel display is described in U.S. Pat. No. 5,541,473 titled GRID ADDRESSED FIELD EMISSION CATHODE that is incorporated herein by reference as background material. Typically, the backplate is formed by depositing a cathode structure (electron emitting) on a glass plate. The cathode structure includes emitters that generate electrons. The backplate typically has an active area within which the cathode structure is deposited. Typically, the active area does not cover the entire surface of the glass plate, leaving a thin strip that extends around the glass plate. Electrically conductive traces extend through the thin strip to allow for connectivity to the active area.
Prior art flat panel displays include a thin glass faceplate having one or more layers of phosphor deposited over the interior surface thereof. The faceplate is typically separated from the backplate by about 0.1 to 2 millimeters. The faceplate includes an active area within which the layer (or layers) of phosphor is deposited. A thin strip that does not contain phosphor extends from the active area to the edges of the glass plate. The faceplate is attached to the backplate using a glass seal.
In one prior art process, glass frit bars are placed within the thin strip in a frame-shape such that the glass frit bars surround the active area of the faceplate. The backplate is then placed over the faceplate. The flat panel display assembly is then aligned and tacked so as to hold the faceplate and the backplate in their proper alignment. Typically, four tacks are used, one in each corner of the flat panel display assembly. The thickness of the frit bars is less than the distance between the faceplate and the backplate such that there is a gap between the top of the glass frit and the bottom of the faceplate. This gap is typically about one to two thousandths of an inch.
The assembly is then placed in an oven and heated to the bias temperature of the glass frit bars (this is done to minimize stress fracturing resulting from the sudden increase in temperature). A laser is then used to melt the glass frit. The heat of the laser melts the glass frit locally and causes the glass frit to expand such that the glass frit contacts the backplate, wetting the surface of the backplate and forming a "bead." The laser is moved, drawing the bead around the surface of the glass frit until the desired seal is formed.
The melting of the glass frit forms an enclosure that is evacuated so as to produce a vacuum between the active area of the backplate and the active area of the faceplate. In operation, individual regions of the cathode are selectively activated to generate electrons which strike the phosphor so as to generate a display within the active area of the faceplate. These flat panel displays have all of the advantages of conventional CRT displays but are much thinner.
Prior art flat panel display fabrication processes often result in a defective seal between the faceplate and the backplate. Defective seals result from imprecise placement of glass frit bars. When glass frit bars are not properly placed, air is trapped between adjoining glass frit bars. This air forms an air bubble that later can rupture, causing a defect. In addition, defects result from the movement of glass frit bars during the laser heating process. That is, as the bead moves across the area to be sealed, friction from the movement of the bead causes movement of the glass frit bars. The movement can result in a defective seal and can cause the glass frit seal to intrude on the active areas of the faceplate and the backplate. In addition, defects occur as a result of movement of glass frit bars when the assembly is being placed into the oven.
Prior art methods for manufacturing glass frit bars are expensive and time consuming. In a typical prior art process for manufacturing glass frit bars, glass frit and organic material are mixed together. A ball mill grinding process is used to obtain the required mixing. Prior art process for mixing glass frit and organic compound requires 16 hours or more of ball mill grinding. The grinding process imparts high levels of contaminants into the resulting glass frit mixture. More specifically, in ball mill grinding processes that use, for example, alumina balls and an alumina jar, the grinding process causes alumina pick up. That is, the alumina balls wear away, imparting impurities into the glass frit mixture. The impurities degrade the sealing glass quality and make the glass frit mixture susceptible to crystallization during the heating process. This crystallization degrades the quality of the resulting seal. In addition, the alumina contamination causes a rise in the glass transition temperature of the glass frit mixture. The contamination also introduces uncertainty into the manufacturing process since the amount of contamination varies from lot to lot. In addition, the grinding process is expensive due to the time involved and the need to purchase and dispose of alumina balls.
Once the glass frit and binder mixture is blended, tape casting methods are used to form thin tape-cast sheets of glass frit mixture. Once a sufficient number of layers of tape-cast sheets are laminated together to form an assembly having the proper height, the assembly is placed in an oven and is heated (typically at about 350-400 degrees Centigrade). This heating process removes the organic compounds in the binder layer. In addition, the heating process sinters the glass frit mixture. The heating process typically only removes some of the organic compounds, resulting in residual impurities. Typically, residual impurities range from 170-220 parts per million (ppm). The resulting glass frit bar is lapped or ground to the desired thickness. In one prior art process, a thickness of about 50 thousandths of an inch (mils) is obtained. Next, the block is cut into glass frit bars having the desired dimensions. In one prior art process, glass frit bars having a width of 137 mils are obtained in lengths adapted to conform to the size of the flat panel display being manufactured.
The glass frit fabrication process is time consuming and expensive due to the numerous fabrication steps. In addition, the residual impurities outgas during the sealing process. Also, the residual impurities increase the melting temperature of the glass frit bars, thus requiring a higher temperature sealing process. These high temperatures required during the sealing process damage the emitters so as to degrade the cathode. Also, the high temperatures induce stress in the surfaces of the faceplate and the backplate. Moreover, the high temperatures cause the surfaces of the flat panel display to outgas. The outgassed contaminates degrade the emitter surface causing electron emissions to be unstable and to be generally reduced. In addition, ions formed through the collision of electrons with outgassed molecules can be accelerated into the emitter tips and may degrade their emission. Plasma formed in the same manner can short emitter tips to the overlying gate and can cause arcing at high field regions in the display. Thus, outgassing interferes with the operation of the cathode, resulting in reduced picture quality.
What is needed is a flat panel display and a method for making a flat panel display that has an effective seal between the faceplate and the backplate. More particularly, a way to make a flat panel display is needed that will reduce defects in the seal. In addition, a flat panel display and a way to make a flat panel display is needed that minimizes damage to the active areas of the faceplate and the backplate. Also, a flat panel display and method for forming a flat panel display is needed that reduces manufacturing time and manufacturing costs. A glass frit bar and a method for forming a glass frit bar having low amounts of impurities is needed. In addition, a need exists for a glass frit bar and a method for forming a glass frit bar wherein the method and glass frit bar are less expensive and reduce fabrication process time. The present invention meets the above needs.